1. Field of the Invention
The present invention relates to a friction material for use in vehicle. More particularly, the present invention relates to a friction material for vehicle brakes which generate less brake noise, particularly less creep noise when an automatic car starts to run.
2. Description of the Related Art
As a friction material mainly for vehicle brakes or the like, there has been recently used one having a porosity from 10% to 30% for the purpose of improving fade resistance and enhancing effect at high velocity.
When a vehicle is operated in rainy weather or runs over puddles, the friction material gets wet with water. Further, when a vehicle is parked outside at night, the friction material can easily absorb water.
The friction material thus moistened generates a low frequency noise when the vehicle is braked and runs at velocity of not more than 5 km/h shortly before stopping while it is being dried. The friction material thus moistened also generates such a low frequency noise when an automatic car starts to run at a very low velocity, i.e., during the full release of the brake pedal which has been stepped after switching to D range, causing the brake to drag due to creep force. Such a low frequency noise is also called "creep noise". This noise offends the operator and thus makes a nuisance of the operator.
The greater the porosity of the friction material is, the more is the water absorption of the friction material and the greater is such a low frequency noise. Further, when the friction material is moistened, squeal is generated, or the friction coefficient is excessively raised, causing the generation of abnormal effect.
Claiming that a friction material which has been impregnated with a water repellent material so that is rendered water repellent can be prevented from forming an extremely thin water film thereon to inhibit a phenomenon called jada during clutch meet in respect to clutch facing, a dry friction material obtained by impregnating a friction material matrix made of organic fiber, inorganic fiber, metal wire, filler and binder with a water repellent material has been proposed (Japanese patent published unexamined application Hei. 4-234479).
According to the above cited application, the water repellent material to be used for this purpose is preferably a material which exhibits water repellency but doesn't impair friction characteristics, such as silicone oil and fluororesin.
The friction material which has thus been rendered water repellent can exert its desired effect on troubles as occurring when a vehicle is parked at night to cause the friction pad to get moistened. However, such a friction material generates greater creep noise when the vehicle starts to run with the rotor and friction pad being directly wet with water due to car washing or the like than friction materials which have not been rendered water repellent, bringing about a contrary effect.